When you write an OO code, you describe object features (attributes, properties, methods) and implement them. This may be seen as declarative: you do not have an imperative approach giving the sequence of operations to reach a given goal; you have a declarative approach describing features of objects, in a sense characterizing them.
To be more precise, let me take an example. When one says that an image has a filtered version, a blurred version, etc, this may be seen as declarative; these are features of any image. When one implements a procedure to filter or blur an image, then this is imperative: the code is thought of as a sequence of operations aiming at transforming an image, not a description of what the transformed image is.
I am not an expert, though, so I hope someone will correct this if needed.